Helical piles, also known as screw piles or screw anchors, are structural, deep foundation elements used to provide stability against forces exerted by axial compression, tension and/or lateral loading (Bradka, 1997). Typical helical piles utilize one or more helical screw plates affixed around one end, the toe end, of a continuous central shaft of smaller diameter with a connection plate at the opposite or top end. Multiple helices used on the toe end of a central shaft can be of equal diameters or have a smaller diameter towards the pile bottom. Helical piles are usually, but not exclusively, fabricated from steel that can also be galvanized for extra protection against corrosion. Helices are attached to the shaft generally by welding, but may also be bolted, riveted, or monolithically made with the shaft (Bradka 1997).
In use, a helical pile is, basically, screwed into the soil, such that the helical plate engages with the soil to distribute the axial load. As a result, there is minimal or no vibration associated with the installation of helical piles, unlike most driven piles. Further, the helices are configured for soil displacement rather than soil excavation, so there is little or no spoil to be removed, eliminating the potential issue of contaminated soils being brought to the surface. This method also engenders tensile strength to screw pile.
Once a helical pile has been placed, it is usually standard practice to test the static load-bearing capacity before beginning other related construction. Conventional “top down” load testing is the most common method of predicting the axial load capacity of helical piles. With this method, a hydraulic jack is positioned against the top end of the pile and works against a loading frame constructed of heavy beams and reaction piles. By observing the top end of the pile, the load vs. deformation behaviors can be recorded during the test and used to predict the capacity of the helical pile. However, this testing method is dangerous, expensive, and can be inaccurate.
About 25 years ago, Dr. Jorj Osterberg developed an innovative, relatively low-cost, alternative static load testing method, referred to as the Osterberg Cell® or O-Cell® for short (Osterberg, 1998). The O-Cell® is a bi-directional static load testing device useful with drilled shafts, bored piles, caissons, driven piles, slurry walls, barrettes, continuous flight auger (CFA) piles, or other similarly constructed pile foundations. The O-Cell test operates by the separation and observation of the shaft and toe behavior, as well as other results important for assessing the adequacy of the pile (Fellenius, 2001). In the conventional bi-directional load test, a sacrificial O-Cell™ (hydraulic jack) is placed during construction of the pile at or near the bottom of the pile. When the pile construction is complete, the previously positioned O-Cell® can be utilized to separate and test the pile loading capacity. During the O-Cell® test, load increments are applied to the pile by means of incrementally increasing the hydraulic pressure in the O-Cell®, which causes it to expand in a jack-like fashion, pushing the pile shaft upward and the pile toe downward (Fellenius, 2001).
Unfortunately, because of the continuous central shaft, necessary to withstand the higher torque required for turning helical piles into the soil, bi-directional technology, such as the O-Cell®, has not been incorporated with helical piles. Thus, “top down” load testing has been the method used for predicting the axial load capacity of screw piles.